This invention relates to hydraulic circuit systems of construction machinery, and, more particularly, to an apparatus for locking an inertial mass drive hydraulic system which controls actuator means for driving a relatively large inertial mass, such as a hydraulic motor for driving a swing of a hydraulic excavator.
A proposed inertial mass drive hydraulic circuit system for construction machinery such as, for example, an excavator, includes a hydraulic pump, actuator means driven by the hydraulic pump for driving a relatively large inertial mass, a directional control valve mounted in line means connecting the hydraulic pump with the actuator means replenishing lines for avoiding production of a negative or subatmospheric pressure in the actuator means, and relief valves for setting a maximum pressure for the main lines between the actuator means and the control valve.
In the proposed hydraulic circuit system, when the directional control valve is switched from one position to another by a control lever to drive an inertial mass, the hydraulic fluid in the hydraulic pump is led to the actuator means such as a hydraulic motor, and the inertial mass is driven by the hydraulic motor. At this time, since force of inertia produced by the inertial mass acts on the hydraulic motor, the speed of the hydraulic motor does not rise quickly, and a high pressure is produced in the main line on the supply side. When the high pressure exceeds the value set by the relief valve, the pressure is released to the main line on the discharge side through the relief valve. Upon completion of acceleration, the hydraulic motor shifts to a steady state condition in which it operates at a constant speed. If the control lever is manipulated to return the control valve to a neutral position, then the main lines are closed by the control valve. At this time, the hydraulic pump continues its operation by the force of inertia of the inertial mass and discharges the fluid drawn from the main line on the supply side to the main line on the discharge side thereby raising the pressure in the discharge side main line. When the high pressure exceeds the value set by the relief valve, the hydraulic fluid in the main line on the discharge side is released to the main line on the supply side through the relief valve. Thus, the hydraulic motor is gradually decelerated by the high pressure in the main line on the discharge side. At this time, as the pressure in the main line on the supply side tends to become subatmospheric, hydraulic fluid is replenished to the main line through the replenishing lines from a reservoir. The aforesaid description also applies to the operation of reversely switching the directional control valve to cause the hydraulic motor to operate in the reverse direction.
One disadvantage of the proposed drive hydraulic circuit system resides in the fact that, when, for example, the hydraulic motor for driving the swing of a hydraulic excavator is driven by the inertial mass drive hydraulic circuit system, the swing would tend to rotate downwardly by its own weight and apply an external force to the hydraulic motor if the hydraulic excavator is stopped on a sloping ground and a prime mover for driving the hydraulic pump is rendered inoperative. If the directional control valve is actuated by mistake in a direction in which the swing is driven in the downward direction, then the main line on the discharge side of the hydraulic motor would be communicated with the reservoir while the main line on the supply side of the hydraulic motor would be communicated with the discharge side of the hydraulic pump through the supply line and with the reservoir through the replenishing lines. Thus, the fluid flowing through the hydraulic motor would flow in circulation and cause the swing to move in swining movement by the operation of the hydraulic motor. This means that when the hydraulic motor (actuating means) for driving the large inertial mass has the weight of the inertial mass itself or an external force applied thereto, there would be the risk that the hydraulic pump would be actuated if the directional control valve is switched by mistake in a direction in which the hydraulic motor is driven by the weight of the inertial mass or an external force after the hydraulic pump is rendered inoperative by stopping the prime mover.
In the hydraulic circuit system of the aforesaid construction, it is known to provide counter-balance valve means interposed in the main lines between the directional control valve and the actuator means without use of replenishing lines. The counter-balance valve means has a locking function to keep the actuator means from being actuated by its own weight or an external force as the directional control valve is operated by mistake when the hydraulic pump is not in a discharge condition, in addition to the subatmospheric pressure preventing function similar to that performed by the replenishing lines, and the function of keeping the pressure in the supply side main line at a predetermined level. Thus, the hydraulic circuit system provided with the counter-balance valve means is able to avoid the aforesaid risk by its locking function. However, the counter-balance valve means suffers the disadvantages that the valve is complex in construction because of its compound functions described hereinabove and that the valve causes a large loss of energy as by restricting of the fluid because of a built-in check valve. Also, the counter-balance valve means might become operative due to a change in pressure during operation of the hydraulic circuit system irrespective of the wishes of the operator, thereby rendering the operation of the system unstable. Moreover, the system becomes expensive because the construction is complex.